A brushless direct current, (Direct Current, DC) motor is a major type of motor, and a basic structure of the existing brushless DC motor is shown in FIG. 1, which shows a schematic cross-sectional diagram of a stator in a brushless DC motor in the prior art. In a cross section of the stator perpendicular to an axis of a rotating center of the rotor, including a stator 1 having paired stator teeth, a rotor 2 having paired magnetic poles and a rotating shaft fixed in the rotating center 3 of the rotor, where each of the stator teeth 11 is wound with a winding. The stator 1 includes a stator outer circle and stator teeth 11, each of the stator teeth 11 consists of a tooth root and a tooth top. The tooth root, of which the shape is of axially symmetric plate-like, extends from the stator outer circle towards the rotating center 3 of the rotor, and two sides of the tooth top respectively extend from two sides of the tooth root to form a slot 13 of a cogging 12; the center of the tooth top arc 111 for each stator teeth 11 is located at the rotating center 3 of the rotor. The working principle of the brushless DC motor is that, a driving current commutated according to a set period is applied to the winding to produce a changing magnetic field between the stator teeth and rotor magnetic poles, such that an electromagnetic torque is produced on the rotor to drive the rotor to rotate.
There is a general defect in the prior art, that is, in a half commutating period of the brushless DC motor, both the electromagnetic torques are zero when the phase angle between the magnetomotive force of the stator and the magnetic flux of the rotor is 0 degree and 180 degrees. Therefore, these two positions are called “dead points”, when the rotor is in the vicinity of the “dead point”, the starting torque of the motor acting on the rotor is very small, and thus the motor is difficult to start. With respect to the defect in the prior art, in the commonly-used four types of stator structures (graded air-gap structure, stepped air-gap structure, asymmetric teeth structure, additional groove structure) with asymmetric air-gaps in the prior art, by acquiring an asymmetric reluctance torque, which generates an oriented torque after combined with a permanent magnetic torque, a total resultant torque generated in the vicinity of the dead point appears to be a positive torque, so as to avoid the “dead point”. However, all these asymmetric air-gap structures are provided by changing the shape of the tooth top arc 111 of the stator teeth 11. For details, please refer to FIGS. 2-5, in the graded air-gap structure, different tooth top arcs of the stator teeth have the same center but have different radii R11, R12, R13, R14 and R15; in the stepped air-gap structure, the tooth top arc surface of the same stator tooth forms a suddenly changed step t; in the asymmetric teeth structure, the symmetry axis k2 of the tooth top cylindrical surface of each stator tooth deviates from the symmetry axis k1 of the stator tooth; in the additional groove structure, a groove h is provided on one side of the symmetry axis of the tooth top cylindrical surface of each stator tooth.
Since the center axes of the tooth top cylindrical surface of the stator tooth in all four types of stator structures in the prior art lie in the rotating center of the rotor, such that sudden changes exist in the air-gaps between different stator teeth and the rotors, causing the reluctance torque to have a positive peak and a negative peak within a pole pitch, namely, showing a high torque ripple, making vibration of the motor increased and meanwhile a power density of the motor reduced, thus the power of the motor is small and the efficiency is not too high.